The long-term objective of this research is to discover how the development of nerve cells proceeds. I want to study, at the phenomenological level, the chain of events that leads from a presumptive nerve cell to a fully differentiated neuron. I have developed an in vivo and in vitro preparation of amphibian spinal neurons in which the development of morphological and functional characteristics can be studied. The ionic basis of the inward current of the action potential of these neurons has been analysed with electrophysiological techniques, and changes during development, from Ca ions to Ca ions and Na ion to Na ion, both in the embryo and in culture. An investigation of the chemical excitability of the same neurons has been undertaken, and the sensitivity of neurons to bath application of transmitters in vivo and in vitro has been determined. This preparation will be used in the further analysis of neuronal development. The project is designed to answer some questions about the chemical excitability of embryonic neurons. Present evidence suggests that depolarization by GABA may be a marker by which Rohon-Beard neurons from Xenopus can be identified in vitro. This point will be tested 1) by further characterizing the magnitude, localization, and ionic basis of the responses of these cells in the spinal cord, and of cells depolarized by GABA in culture, to see if they are similar in all of these aspects (these aims will be achieved by intracellular recording of responses to the iontophoretic application of the neurotransmitter); 2) by taking advantage of a technique to extrude the cells from Amblystoma embryos, comparing their chemosensitivity in culture with that in the spinal cord, and then demonstrating that they are absent from the embryos. Subsequently, another population of neurons in culture, that are hypopolarized by GABA and GLY and depolarized by GLU, will be characterized. The properties of these cells, some of which form nerve-muscle junctions, will be compared with those determined for motor neurons in vivo, to see if chemosensitivity can be used to identify this cell type in vitro. With these baselines established, the timing and nature of appearance of transmitter sensitivity in vivo and in vitro will be studied.